1. Technical Field
This invention relates to the art of increasing the wear resistance of exhaust gas recirculation (EGR) valve bushings and valve stems used in internal combustion engines, and particularly to techniques for elevating the operating temperature of such EGR components.
2. Discussion of the Prior Art
The earliest EGR systems used in most vehicles (starting in 1972-73) were designed to reduce emissions of oxides of nitrogen (NO.sub.x). They have also been influenced drivability, octane rating requirements, and fuel economy of some vehicles. The reduction of NO.sub.X is accomplished by lowering engine combustion temperature by recirculating metered amounts of burned exhaust gases back through the intake manifold where such gases are mixed with a fresh air/fuel mixture.
Current EGR valve designs (see U.S. Pat. No. 4,044,737) operate at temperatures in the range of 650.degree.-750.degree. F., permitting use of relatively economical materials for the valve stem (such as stainless steel) and for the bushing (such as bronze impregnated with graphite). With the projected increase in durability standards for automotive components, such current EGR valve design will be expected to survive 50,000-100,000 miles of engine operation with little change in leakage. Such known materials may exhibit excessive wear at the bushing-stem interface for such extended periods.
More importantly, there is a desire to raise the design requirements for EGR valves to intermediate operating temperatures in the range of 800.degree.-900.degree. F. and in certain truck applications to operating temperatures in the range of 900.degree.-1200.degree. F. Such increases in temperature may be brought about by (i) increasing the exhaust gas recirculation flow which is either needed to achieve emission standards and possibly increase fuel economy and thereby help meet federal corporate average fuel economy (CAFE) requirements, or (ii) locating or burying the EGR valve assembly closer to the exhaust manifold.
At such higher operating temperatures, the existing bushings deteriorate dramatically, possibly due to the oxidation of graphite from the impregnated bronze and at even higher temperatures accompanied by the oxidation of the bronze metal; oxidation results in unacceptable wear and valve leakage. There may also be, at such increased exhaust recirculation flows, a tendency for increased deposits on the valve stem which is exposed to such gases; this results from the chilling effect on the stem which is alternately exposed to a relatively cool environment.
Ceramic materials are well known for their wear resistance, tolerance to elevated temperatures, and their hardness. However, ceramics are brittle in tension making them undesirable as valve stem materials; moreover, ceramics do not wear well in sliding engagement with each other nor promote wear with known high temperature metal alloys needed for valve stem constructions such as stainless steel. Thus, there is a clear need for improved material system design of the valve assembly to meet these changing conditions and to permit use of ceramics.